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NATIONAL COLLEGE OF SCIENCE AND TECHNOLOGY Amafel Building, Aguinaldo Highway Dasmariñas City, Cavite ASSIGNMENT # 1 OPERATIONAL AMPLIFIERAlido, Ronald C. July 26, 2011Electronics 3/BSECE 41A1 Score: Engr. Grace Ramones Instructor
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OPERATIONAL AMPLIFIERS- designed to be used with other circuit components toperform either computing functions (addition, subtraction) or some type of transfer operation,such as filtering. Operational amplifiers are usually high-gain amplifiers with the amount of gaindetermined by feedback.Composition and symbols. The earliest IC op amp output stages were npn emitter followerswith npn active loads or resistive pull-downs. Using a FET rather than a resistor can speed thingsup, but this adds complexity. With modern complementary bipolar processes, well-matched highspeed pnp and npn transistors are available. The complementary output stage of the emitterfollower has many advantages, and the most outstanding one is the low output impedance. Theoutput stages constructed of CMOS FETs can provide nearly true rail-to-rail performance. Mostof the modern op amps have the class B output stages of some sort. Fig. 2.18 displays schematic symbols of an op amp. In the first of them, KU is the voltagegain. The inverting input is U 1 , and the non-inverting one is U 2 . U 1, and U 2 are the nodevoltages measured with respect to the ground. The differential input is the difference of two nodevoltages, and the common-mode input is their half-sum. Since the quiescent output of an op amp is ideally zero, the ac output voltage (MPPvalue) can swing positively or negatively. In particular, for high load resistances, the outputvoltage can swing almost to the supply voltages. For instance, if UC = +15 V and UE = 15 V, theMPP value with a load resistance of 10 k or more is ideally 30 V. In reality, the output cannotswing all the way to the value of the supply voltages because there are small voltage drops in thefinal stages of the op amps. In other schematic symbols of an op amp, the plus sign correspondsto the non-inverting inputs and the minus or the rounded inputs are the inverting ones. The frequency range of an op amp depends on two factors, the gain-bandwidth product(voltage gain multiplied by midband) for small signals, and the slew rate for a large signal. Aslew rate of an amplifier is the value of the maximum rate of change of the output voltage pertime. It is usually less than 10 V/s. The slew rate limitation makes op amps unsuitab forleapplications, which require fast-rising pulses. Therefore, the op amps should not be used as thesignal sources of the digital circuitry feed.
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Non-inverting feedback voltage amplifier. As discussed earlier, one of the most valuable ideasof electronics is the negative feedback. In an amplifier with a negative voltage feedback, theoutput is sampled and part of it is returned to the input. The advantages of the negative feedbackare as follows: more stable gain, less distortion, and higher frequency response. In Fig. 2.19, a non-inverting op amp is presented. Here, the output voltage is sampled bythe voltage divider and fed back to the inverting input of the op amp. The differential input of the op amp is an error voltage, defined as The op amp amplifies this error voltage as Uout = KdUerr, where the amplification factor Kd is the differential voltage gain of the open-loop opamp. Let K 1 be the feedback fraction or the fraction of output voltage fed back to the input, thatis K 1 = R 1 / ( R 1 + R 2) = U 1 / Uout. Then, the output voltage is Uout = Kd ( Uin – K 1 Uout). By rearranging, K = Uout / Uin = Kd / (1 + KdK 1). This famous formula defines exactly what the effect of negative feedback is on theamplifier. Here one can see that the voltage gain K of the closed-loop amplifier with negativefeedback is less than the differential voltage gain Kd of the open-loop op amp. The fraction K 1is the key to how much effect the negative feedback has. When K 1 is very small, the negative
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feedback is small and the voltage gain approaches Kd. However, when K 1 is large, the negativefeedback is large and the voltage gain is much smaller than Kd. The product KdK 1 is called a loop gain because it represents the voltage gain going allthe way around the circuit, from the input to the output and back to the input. For the non-inverting voltage feedback to be effective, a designer must deliberately make the loop gain muchgreater than one. Once this condition is satisfied, K = Uout / Uin 1 / K 1 = ( R 1 + R 2) / R 1. The equation states that the voltage gain K of the closed-loop system is equal to thereciprocal of K 1, the feedback fraction, and no longer depends on the value of Kd. Since Kddoes not appear in this equation, it can change with temperature or op amp replacement withoutaffecting the voltage gain. The IC op amps approach such requirements and have extremely high differential voltagegain Kd. When the feedback path is opened, the open-loop voltage gain is approximately equal tothe differential voltage gain. When Uin = 0, Uout = KU 0 , where U 0 is called a zero offset. Inthe case of R 2 = 0 , Uout Uin. This circuit is called a buffer. A buffer does not amplify thevoltage but it can be of high power gain and play the role of an impedance converter. Some circuits require the positive feedbacks. The voltage gain K of the closed-loopamplifier with the positive feedback is more than the differential voltage gain of the open-loopop amp K = Uout / Uin = Kd / (1 – KdK 1). The maximum value of KdK 1 is to be less than one. In an opposite case, the outputsignal will grow and the system may become unstable. Typically, this effect is used in pulsegenerators − pulsers.
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Inverting feedback voltage amplifier. An inverting amplifier given in Fig. 2.20,a also uses thenegative feedback to stabilize the working conditions in the same way. Here, the output voltagedrives the feedback resistor R2, which is connected to the inverting input. The voltage gain isgiven by R2 The circuit characteristic is linear. By virtue of the negative voltage gain, the amplifierinverts the input signal. In the case of R 1 = R 2 the circuit is called an inverter because theoutput signal is equal to the inverted input signal. Its circuit symbol is shown in Fig. 2.20,b.
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Feedback current amplifier. Fig. 2.21 illustrates an amplifier with the inverting voltagefeedback. Here, the output voltage drives the feedback resistor R2, which is connected to theinverting input, and the voltage gain is independent on the error. Instead of acting like a voltageamplifier, an amplifier with an inverting voltage feedback acts like an ideal current-to-voltageconverter, a device with a constant ratio of output voltage to the input current. As Kd is muchgreater than unit, Uout = KdUerr, K = Uout / Iin = KdR 2 / ( Kd + 1) R 2. The ratio Uout / Iin is referred to as a transresistance. Besides stabilizing oftransresistance, the inverting feedback has the same benefits as the non-inverting voltagefeedback that is decreasing distortion and output offset. When R 2 = 0, the current amplifier is a voltage repeater because the voltage gain isequal to unit .
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Feedback differential amplifier. Fig. 2.22 shows an op amp connected as a diff amp with thebalanced supply. It amplifies Uin that is the difference between U 1 and U 2. The output voltageis given by Uout = KUin, where K = R 2 / R 1. When U 2 is zero, the circuit becomes an inverting amplifier withUout(1) = KU 1. When U 1 is zero, the circuit becomes a non-inverting amplifier with K = R 2 / R 1 + 1. When both inputs are presented, Uout = Uout(1) – Uout(2).
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Summary. To provide high efficiency and operational speed, most of the contemporary opamps have the class B output stages. The quiescent output of an op amp is zero and the MPPvalue can swing positively and negatively almost to the supply voltages. Op amps have a broadfrequency range and limited slew rate therefore they are very popular in analog electronics andless preferable in fast-speed digital circuits. To obtain a stable gain, low distortion, and high frequency response, the inverting or non-inverting negative feedbacks are used in the op amp circuits. The higher is the negative feedbackvoltage the lower is voltage gain and the higher the frequency response. The buffers, theinverters, the voltage repeaters, and the diff amps are the useful representatives of the op ampswith a negative feedback.
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